The best thing you can do, as usual, is google it and read documentation.

That said, basically when you want to implement skeletal animation you have two task:

1) define a bone structure

2) attach the bone structure to a mesh

you do 1) either by creating a bone system manually, or with biped

you do 2) either with Physique or Skin

So biped is just an automated way of creating a bone system

Physique and Skin are equivalent, they link the bone system with the mesh. One thing that confused me in the beginning was why autodesk was offering two tools to do the same thing in a very similar way. There is no explanation of this in the doc, but I arrived to the conclusion (I might be wrong) that in the beginning there was Physique. Then autodesk acquired Skin from some third-party, release it but leaving Physique for backward compatibility.